Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high-speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns.
Discrete ratio transmissions are capable of transmitting power via various power flow paths, each associated with a different speed ratio. A particular power flow path is established by engaging particular shift elements, such as clutches or brakes. Shifting from one gear ratio to another involves changing which shift elements are engaged. In many transmissions, the torque capacity of each shift element is controlled by routing fluid to the shift elements at a controlled pressure. A controller adjusts the pressure by sending electrical signals to a valve body.
To save fuel, some vehicles include stop/start programming to automatically shut off the engine when the vehicle is stopped while in Drive, such as while waiting at a red traffic light. The engine is then automatically started when the driver lifts his or her foot off the brake pedal. It is important that the vehicle begin accelerating as soon as the driver presses the accelerator pedal. To accomplish that, it may be necessary to keep one or more shift elements engaged. Since an engine driven pump does not provide pressurized fluid when the engine is off, a different source of pressurized fluid may be included, such as an accumulator.
During various types of driving, it is possible to request a stop/start event when the accumulator is not adequately filled. For example, during city driving it is possible to have repeated stop/start requests without increasing engine speed or driver demand enough to raise a line pressure high enough to fully charge the accumulator. This scenario may generate inconsistent restart engagement times and inconsistent engagement feel since the accumulator is not adequately filled.